In order to supply the pressure required to draw oil from the ground such as when the oil pressure itself is insufficient to lift the oil to the earth's surface, it is conventional to place a pump in the well and to drive the pump by a pumping unit on the earth's surface. The pumping unit is often connected to the pump by a string of interconnected sucker rods known as a sucker rod string. Although steel sucker rods have been widely used, it has more recently been found that lighter weight plastic and fiberglass reinforced plastic sucker rods are more effective since they allow use of smaller motors and pumping units, are not subject to the corrosion on steel rods, and they are less expensive to operate in the field.
Such non-metal rods must be connected in an end-to-end relationship to achieve the length required to connect the pump and the pumping unit, while providing rigidity to this rod string and to enable replacement of sections of the rod string when failures occur rather than requiring replacement of the entire rod string. While steel rods generally have lengths of 25 to 30 feet and diameters of 1/2 to 11/2 inches, plastic and fiberglass reinforce plastic rods may be longer. For example, Bennet Industries markets a standard fiberglass reinforced resin rod having a length of 37 feet 6 inches and an outer diameter of 1 inch. In deep wells whose production tubes, through which the sucker rod string extends, exceed about 3000 feet, a combination of steel rods and fiberglass reinforced plastic rods are now frequently used. Sucker rod fittings are used to connect individual sucker rods. For example, U.S. Pat. No. 4,315,699 discloses a fitting having a high strength, elongated, metal cylindrical retainer and a stack of metal collets whose inner peripheral profile has the shape of a plurality of pairs of truncated cones placed base-to-base, with the end collet abutting the retainer; a potting compound is poured into the space between a received rod and the rings for locking the rod in place. Joslyn Manufacturing and Supply Company of Chicago, Ill. at one time made a fitting from a solid piece of steel having a bore extending from an open end to a closed end; cylindrical surfaces defined sections at opposite ends of the bore, and four intermediate sections were defined by alternately long, outwardly diverging surfaces and short, inwardly converging surfaces. The length of the respective intermediate sections were equal. Fiberflex Products, Inc. of Big Spring, Texas markets a similar product. The latter company is the assignee of U.S. Pat. No. 4,360,288 which discloses a sucker rod fitting or connector having an axial receptacle for a cylindrical fiberglass rod. The receptacle is a blind bore defined by a series of outwardly converging tapered surfaces into which an adhesive material is placed to form, when cured, a series of wedge shaped annuluses about the rod to secure it in the fitting. A collet is attached to the open end of the fitting to align the rod with the fitting.
One of the time-consuming and expensive problems in the field associated with sucker rod fittings is the replacement of sucker rods which have failed. In the case of fiberglass reinforced plastic rods used with the known fittings described above, breakage generally occurs outside of the fitting, and the break is characterized by a broomstraw effect where the plastic or resin and fiberglass strands separate. With such a break, it is a difficult and slow process to grasp the shattered part of the lower part of the sucker rod string and lift it to the surface, especially since it is not uncommon for such breaks to occur 1000 feet or more in the production tube beneath the surface of the earth. Once the part of the sucker rod string with the broken sucker rod is recovered, the rod is replaced by removing the failed rod and inserting the new rod in the couplings at its opposite ends.